Ultrastructural aspects of nuclear differentiation in the immature rat uterus will be followed to determine in detail the time course of the appearance of nuclear bodies in relation to single injections of estradiol, estriol, and Nafoxidine and to an implant of estradiol. The early gestational human endometrium will be analyzed through ultrastructural, enzymic, and carbohydrate cytochemistry to determine the nature of the apparent secretory process of the decidual cells, particularly in terms of lysosomal differentiation. Also the state of activity of the stromal monocytes will be determined through localization of peroxidase and acid phosphatase activity. The ultrastructural features of the luminal and glandular epithelia of the normal human endometrium will be compared during the menstrual cycle and in early gestation. To test the hypothesis that an inflammatory response in a regressing uterus is directly associated with collagen breakdown rather than with the presence of a fetoplacental complex, the cellular differentiation of the uterine stroma will be followed during the regression that occurs in deciduomata created experimentally in pseudopregnant rats.